Cell fate can be specified during development in two fundamentally different ways. In most cases, fate decisions depend on specific signals that produce essentially hardwired, highly reproducible outcomes. In a number of important alternate cases, differentiated cell types arise instead by chance, through stochastic mechanisms. One of the best studied examples of this second strategy is the specification of photoreceptor subtypes in the Drosophila retina. We have identified a related group of flies that instead make the same neural fate decision in a deterministic and highly ordered way, producing alternating single rows of photoreceptor subtypes instead of the random salt and pepper distribution in the Drosophila retina. Objectives: Our preliminary studies indicate that differential regulation of the key transcription factor Spineless is critical in both systems. We propose a comparative approach to characterize upstream and downstream components in the ordered system and to further dissect the context and regulatory control of this decision in both systems. Specific Aims: (1) To characterize retinal patterning in the alternate, ordered system (Dolichopodidae, or Doli) and to test its function; (2) to better understand each how each cell fate determination system is controlled by identifying regulatory regions of critical determinants; and (3) to further dissect photoreceptor subtype specification in Drosophila by applying newly developed genomic tools. Study design: We are sequencing the genome and transcriptome of two species in the deterministic system (Doli). This will allow us to characterize mRNA expression patterns of genes in the retinal patterning network, alongside traditional antibody development for key transcription factors. To assess potentially conserved regulatory function we will test candidate regions from Doli in Drosophila as well as develop transgenic techniques in Doli. To further evaluate the role and function of upstream and downstream components of this fate decision in Drosophila, we will use FACS-sorted, cell-type specific populations of photoreceptor nuclei for transcriptional and chromatin mark profiling. Overall relevance: These studies will contribute to our understanding of how different types of cell fate choice are controlled. The discovery of two closely related but alternate systems provides an ideal opportunity to study the role of underlying biochemical noise in the expression of critical cell fate determinants and how it might be utilized - or limited. Several important components of this cell fate specification system and its targets have been implicated in cancer directly, including the human homolog of Spineless itself. Ultimately, a better understanding of how cell fate is determined and maintained will lead to new strategies for targeting therapeutic agents and drugs to specific cell fate specification pathways when cell fate is mis-specified or lost, as often occurs in cancer and in the progression of genetic diseases.